1. Field of the Invention
Our invention relates generally to microwave radio communications assembly and design, and more particularly to a relatively lightweight, compact, and inexpensive directional microwave filter that can be tuned to provide an elliptic filter function. Such filters have many applications, but are especially useful in frequency multiplexers and demultiplexers for communication satellites.
For purposes of this document, the term "microwave" encompasses regions of the radio-wave spectrum which are close enough to the microwave region to permit practical use of hardware similar to microwave hardware--though larger or smaller.
2. Definitions and Systems Considerations
This document is written for persons skilled in the art of microwave component assembly and design--namely, for microwave technicians and routine-design engineers.
Very generally, a multiplexer is a device for combining several different individual signals to form a composite signal for common transmission at one site and common reception elsewhere. Typically the several individual signals carry respective different intelligence contents that must be sorted out from the composite after reception; hence the multiplexing process must entail placement of some kind of "tag" on the separate signals before combining them.
The multiplexers of interest here are frequency multiplexers, in which the "tag" placed upon each signal is a separate frequency--or, more precisely, a separate narrow band of frequencies. Each signal is assigned a respective frequency band or "channel" and is transmitted only on that band, but simultaneously with all the other signals.
After reception the several intelligence contents are resegregated (demultiplexed) by isolating the components of the composite signal that are respectively in the assigned frequency bands. Each intelligence stream is thus directed to a respective separate device for storage, interpretation, or utilization.
In satellite operations the transmission is by radio through the ether, and all the signals are transmitted through a common antenna. Operations in the microwave region (as defined above are most customary.
A microwave frequency multiplexer generally consists of several frequency-selective devices, termed "filters," positioned along a combining manifold. Such a manifold is essentially a pipe or "waveguide" of rectangular or circular cross-section, through which microwave radiation propagates in ways that are well-known to those skilled in the art--namely, microwave technicians and design engineers.
Separate sources of intelligence-modulated but usually broadband microwave signals respectively feed the filters. "Broadband" means spanning a frequency band that is considerably broader than the narrow band assigned to each intelligence channel. Usually each source feeds its respective filter through another short piece of waveguide.
The details of generating these broadband signals and modulating them with intelligence that is to be transmitted, as well as the details of the transmission and reception process, are outside the scope of this document. The means used for demultiplexing after reception, however, are within the present discussion. At least in principle, most multiplexers if simply connected up in the reverse direction act as demultiplexers. As will be seen, however, demultiplexers for ground stations or for very large craft are not subject to such mass and size constrainsts as demultiplexers for communications satellites. For simplicity in most of the discussion that follows, we refer only to multiplexers.
Each of the several filters in a multiplexer is assigned a frequency band generally different from that which is assigned to all the others. Each filter is constructed and adjusted so that it permits most of the microwave radiation within its band to pass on into the manifold--and so that it stops most of the radiation outside its band (in either direction along the frequency spectrum). These two frequency categories with respect to any particular filter are accordingly sometimes called the "pass band" and "stop band" of the filter.
Design requirements for multiplexers on spacecraft include several constraints which have been extremely difficult to satisfy in combination. Although particularly troublesome in communications repeater satellites and the like, many of these constraints are common to multiplexers and filters generally, as will be seen.
First, it is highly desirable to minimize the overall weight and bulk of spaceflight equipment, with reasonably low cost. This consideration is particularly important to bear in mind because heretofore the best solution for most of the other constraints in this field has required such high overall weight, bulk, and cost as to be completely unacceptable.
Second, it is highly desirable to minimize both the overall use of electrical power and the dissipation of electrical power as heat within communications components. The overall power to the communications system must be supplied from the spacecraft power supply, which is severely limited. Overall power includes not only the desired output power to the antenna, but also the dissipation losses in components, including filters. Moreover, each instance of significant heat dissipation complicates the overall thermal-balance design of the craft. Both these considerations favor components, including filters, that dissipate very little power. In other words, it is preferable to use filters with very high "Q" or quality.
Third, it is desirable that all of the sources make essentially equal power contributions to the composite signal. Otherwise the overall power to the antenna must be increased as required to transmit the weakest channel stream with an adequate ratio of signal to background noise, and this increase wastes power in all the other channels.
This channel-equalization consideration is very closely related to the low-dissipation concern discussed above, but only in certain cases. The operating principle of some filters requires a multiplexer layout in which the output of one filter passes through other "downstream" filters en route to the antenna. In such a multiplexer the dissipation which each other filter imposes upon the signal from the upstream filter is cumulative. Signals from upstream filters are subject to more power loss in dissipation than signals from downstream filters. Consequently to the extent that the individual filters are dissipative the source power in different channels is differently attenuated, or unequalized, in approaching the antenna.
Channel equalization is of relatively small importance, because inequalities in the coupling between each source and the antenna can be compensated by adjusting the power outputs of all the sources. Nonetheless, a practical convenience of some value is obtained by using a multiplexer system that intrinsically produces interchannel power equalization. Some filter types have this property intrinsically and others do not.
Fourth, symmetrical distribution of both weight and thermal dissipation is very desirable in spacecraft. Without such symmetry the control of maneuvers and of thermal balance are more severe problems. These considerations not only accentuate the desirability of low overall weight, low overall electricity consumption and low dissipation in individual components, but also place a premium upon the designer's freedom to position sizable electronic components arbitrarily. Hence it is desirable to be able to position multiplexer filters at will along the multiplexer manifold. Such arbitrary positioning is possible with certain kinds of filters but not others, as will be detailed below.
Fifth, it is extremely desirable to provide filters that can be both positioned and tuned independently of one another. Otherwise installation and adjustment are an extremely delicate, protracted and sometimes iterative procedure, contributing significantly to the overall cost of the apparatus. Here too, certain types of filters are nearly independent of their neighbors along a multiplexer manifold, while other types are not.
Sixth, in virtually all spacecraft communications applications, practical economics requires providing as many communications channels as possible within the overall waveband of the spacecraft transmitter. This condition has led to routine specification of rather narrow wavebands for each channel, and even more significantly to very narrow "guard" bands--unused frequency bands that separate the channels to avoid crosstalk between adjacent channels. In other words, close spacing of frequencies in the frequency-multiplexer overall frequency band is nowadays a fixed requirement.
Consequently filters must be used that provide good isolation of adjacent channels even though their spacing in the frequency spectrum is very slight. This means that it is necessary to inquire into the precise manner in which the signal-passing properties of a filter change with frequency. If the transmission of a filter is plotted against frequency, the resulting graph or curve illustrates the "filter function" or "shape" or "cutoff characteristic" of the filter. These are of crucial importance.
Ideally such a graph shows very high values of transmission within the passband and very low values elsewhere. Further, in such a graph the lines at both edges of the passband, connecting the high-transmission portion of the characteristic curve in the passband with the low-transmission portions elsewhere, ideally are almost vertical. In other words, the ideal filter provides a very sharp "cutoff."
Of course the same ideas can be expressed in terms of a graph of attenuation vs. frequency: the ideal filter function shows very low values of attenuation in a "notch" region defining the passband, very high attenuation at both sides, and essentially vertical lines representing the sharp cutoff characteristic at both sides of the notch.
Certain types of filters, but not others, provide adequate attenuation and adequately sharp cutoff for satellite microwave communications.
3. Prior Art
A basic microwave filter consists essentially of a resonant chamber--typically a metallic cylinder, sphere, or parallelepiped--that is made to support an electromagnetic standing wave or resonance in the contained space.
As is well-known, electromagnetic energy at any frequency has an associated wavelength and tends to resonate in a chamber whose dimensions are appropriately related to that wavelength. A filter chamber or cavity is constructed to approximately correct dimensions for a desired resonant frequency and is then tuned, generally by adjustment of tuning "stubs" or screws that protrude inwardly into the chamber, to vary the electromagnetically effective dimensions.
A single resonant cavity, when used to support within it a single electromagnetic resonance, works only in an extremely narrow band of frequencies. In the ideal "lossless" resonator the frequency band is theoretically infinitesimal. In any practical resonant chamber, however, there are some losses--due to electrical conduction induced in the chamber walls by the electromagnetic fields in the contained space--and associated with these losses is a very slight broadening of the frequency band of the individual resonating chamber.
If broadband microwave power is introduced into such a chamber (through an entry iris, for instance) whatever portion of the input power is oscillating at frequencies within the frequency band of the chamber will "excite" the chamber. In other words, such power is capable of accumulating as energy in an electromagnetic standing wave within the chamber. Some of this energy may be drawn out of the chamber (through a suitably positioned exit iris, for instance) as narrowband power. Whatever portion of the input power is oscillating at frequencies outside the frequency band of the chamber will not excite the chamber significantly, and cannot be drawn off in significant quantities. The chamber simply rejects such vibrations.
Taking a conceptual overview of such a chamber (and its two irises, or equivalent input and output features), the chamber operates as a filter--permitting only power in a narrow frequency band to pass from entry to exit. A standard treatise describing the theory and some practical procedures for assembly and adjustment of microwave filters is Matthaei, Young and Jones, Microwave Filters, Impedance-Matching Networks, and Coupling Structures (McGraw-Hill 1964, reprinted Artech House, Dedham Mass. 1980). A useful reference work is Saad, Hansen and Wheeler, Microwave Engineers' Handbook (two volumes, Artech House 1971).
In practice two or more such chambers are generally assembled to form a series of resonators. If the individual chambers are tuned to slightly different frequencies, the overall assemblage supports a resonance that is slightly degraded but that extends over a frequency range which is significantly broadened, encompassing the two or more frequency ranges of the different chambers. This broadening may be useful in various ways--for instance, to accommodate frequency drift with temperature, or Doppler shifts due to relative velocity of transmitter and receiver.
Broadband microwave power may then be introduced into, for example, one end of the series of chambers, and that portion of the power that is oscillating at a frequency within the broadened passband can be drawn away from, for example, the other end of the series of chambers.
The technique used for coupling power from a filter to a manifold or other waveguide is very important to multiplexer performance. Before 1957 the best available arrangement was the "short-circuited manifold." This technique made use of a well-known property of resonator cavities, not only electromagnetic but also acoustic and other types. A solid wall can be placed completely across such a chamber without interfering with the resonance, provided that the wall is positioned at a "node" of the resonance--in other words, at a point where the standing wave is always zero anyway.
This condition is satisfied, for example, by "driving" the resonance (pumping energy in) at a distance of one-quarter wavelength from the wall, where the corresponding standing wave should have a maximum. Several resonances at respective different frequencies can be established in the same resonator by supplying the driving energy at the corresponding quarter-wavelengths from the end wall. Such multiple resonances can be present one at a time, or--with certain modifications--simultaneously.
In the microwave field an end wall is electrically a short circuit; hence the term "short-circuited manifold." To form a multiplexer using this configuration, each filter must be positioned, in effect, a quarter-wavelength from the short-circuiting end wall. Since different frequencies correspond to different wavelengths, the various filters are at slightly different distances from the wall.
This elementary configuration has several advantages. For one, no extra components are required to couple the filters to the manifold. Weight, bulk and cost therefore are moderate, and can be minimized by modern techniques which use each chamber for two or even three different resonances--"dual mode" or "tri mode" cavities.
Though dual-mode filters were proposed by Ragan in 1948 (Microwave Transmission Circuits, MIT Radiation Laboratory Series 9 673-77, McGraw-Hill), a first practical realization of such filters seems to have been introduced by Atia and Williams, in a paper entitled "New Types of Waveguide Bandpass Filters for Satellite Transponders," Comsat Technical Review 1 21-43 (fall 1971).
Similarly, tri-mode filters were described by Currie in 1953 ("The Utilization of Degenerate Modes in a Spherical Cavity," Journal of Applied Physics 24 998-1003, August 1953), but a practical two-cavity tri-mode filter remained to be disclosed by Young and Griffin in U.S. Pat. No. 4,410,865, issued in 1983.
In multiplexers using the short-circuited-manifold technique the dissipation is also low, and very little of the power from each filter passes through any of the other filters; hence there is no serious interchannel power imbalance.
Thus the short-circuited-manifold technique performs satisfactorily with respect to the first three considerations discussed in the preceding section.
Furthermore, the short-circuited-manifold technique is amenable to extremely sophisticated modern methods for shaping the attenuation notch of each filter. These methods provide sharp cutoffs and thereby permit very narrow guard bands.
More specifically, these methods entail providing not just one sequence of couplings between the multiple resonances in a series of resonant chambers, but two or even several different "routes" from one resonance in the series to later resonances. The complete series, taken one step at a time from the entry resonances to the exit resonance, is usually called the "direct" coupling sequence. Some couplings in these modern systems, however, jump across what could be called "shortcuts" between two resonances in the direct-coupling sequence. These couplings are usually called "bridge" couplings.
When the bridge couplings are suitably designed, they produce resonances that are in the same orientation and location as those produced by the direct couplings, and of nearly equal amplitude, but exactly out of phase. The sum of these two resonances is a single standing wave of very small amplitude--or, in other words, a single resonance that is very strongly attenuated. The diametrical phase difference is thus used to construct a transmission node--an attenuation maximum--in the response of the overall cavity assemblage. In practice, not one but two such attenuation maxima are forced to occur at certain frequencies immediately adjacent to the minimum-attenuation notch. In this way a very sharp cutoff is sculpted at each side of the notch.
Details of these bridge-coupling techniques are set forth clearly in the above-mentioned disclosures of dual- and tri-mode filters, and in other works. The sharp cutoffs achieved are generally called "elliptic" filter functions, since the mathematical functions known as "elliptic functions" can be used to construct the corresponding graphs. Similar performance, however, can also be obtained with "quasi-elliptic" filter functions. These are polynomials arbitrarily constructed by numerical methods; their coefficients do not correspond to any established mathematical function, but are selected simply because they yield the desired microwave filtering results.
The short-circuited-manifold technique thus performs admirably in regard to the sixth consideration discussed above, as well as the first three. It does, however, present two major problems.
First, the filters in a short-circuited-manifold multiplexer are necessarily fixed in location relative to the short-circuiting wall, and in practice they are very close to one another. Symmetrical weight and dissipation distribution of a unitary multiplexer is therefore impossible.
Further, and even more troublesome, the operation of each filter is perturbed by the operation of all the others, so that the actual distance of each filter from the end wall must be an "effective" quarter-wavelength that differs substantially from the distance for that filter operating alone.
These effective quarter-wavelengths must be worked out either by a theoretical analysis (which is typically subject to variation in the actual hardware or by an iterative process of adjusting and readjusting all of the filters in turn. Even when that has been done, variations in the relative operating levels of the sources in the several channels can change the effective quarter-wave positions. Consequently the best solution is only a sort of compromise for typical or average operating levels.
Positioning and tuning independence, as well as symmetrical weight and dissipation distribution, is therefore unavailable in this otherwise useful technique. Many workers have sought a configuration which could provide the missing advantages.
In 1957 Conrad Nelson introduced a "new group of circularly polarized microwave cavity filters" which in fact possessed these advantages ("Circularly Polarized Microwave Cavity Filters," IRE Transactions on Microwave Theory and Techniques, Apr. 1957, 136-47).
When properly positioned relative to an input waveguide through which suitable electromagnetic radiation is propagating, a Nelson filter receives circularly polarized radiation from that waveguide through an entry iris. A Nelson filter also presents circularly polarized radiation of the same sense at an exit iris.
It does so, however, in a frequency-selective manner Speaking generally, radiation that is within the frequency "passband" of such a filter is coupled through the filter appearing as circularly polarized radiation at the exit iris, but other radiation is simply rejected at the entry iris and continues along the input waveguide.
When an output waveguide is also properly positioned at the exit iris, there is established in the output waveguide a propagating radiation pattern that has the same direction of propagation as the source radiation in the input waveguide.
Hence Nelson provided a three-port device. Broadband radiation enters along one waveguide from one direction (the "origin" end of the input waveguide serving as an input port), and radiation in the stop band continues straight along the same waveguide in the same direction the "destination" end of the same waveguide guide serving as an output port). Radiation in the pass band takes a dogleg "jog" (and in some configurations turns a corner) and leaves the filter through a second waveguide, which serves as an output port. Since the direction of propagation in all three ports is completely defined, such a filter is often called a "directional" filter.
Four key facts make Nelson's filter practical. First, on the broad face of nearly every rectangular waveguide there are two lines, parallel to the length of the guide, which represent positions of circular polarization inside the guide. These loci are spaced a known and readily measured distance from the narrower face of the guide. Appropriately shaped irises drilled through the broad face of the guide at any point along either line will tap circularly polarized radiation out of the waveguide.
Second, circularly polarized radiation coupled into Nelson's filter cavity through an iris in the cavity wall can be resolved into its two constituent linearly polarized components for purposes of establishing standing wave structures within the cavity.
Third, these linearly polarized components can be recombined at another point on the cavity wall to resynthesize circularly polarized radiation, which in turn can be tapped out of the resonant cavity through an iris at this other point into an output guide.
Fourth, the circularly polarized radiation can be coupled into another waveguide along one of the circular-polarization loci to reconstruct a propagating wavefront representing power flow along the guide.
Now as to multiplexer construction, several of Nelson's filters can be laid out with a single continuous manifold pipe serving as the output waveguide for all of the filters in common. The several filters all feed this single continuous waveguide in parallel. The power from all of the filters accordingly comes together for the first time in the combining manifold. Power for each channel thus passes through only one filter.
Most properties of Nelson's directional filters are highly favorable for applications of interest here. In particular, these filters have exceedingly low weight, bulk, cost, and electrical dissipation (high Q).
If it were necessary to pass power for some channels through filters for other channels, interchannel equalization using Nelson's directional filters would nevertheless be good, since their dissipation is so low. Not even this minor imbalance, however, is incurred since power for only one channel passes through each filter proper.
Power for all of the channels--whether they are upstream or downstream along the manifold--at most merely passes by the exit irises of filters for other channels. In these transits there is essentially negligible coupling to those other filters and negligible power loss. Interchannel equalization is therefore an intrinsic advantage of the Nelson directional filter.
Furthermore, the Nelson filter may be positioned at any point longitudinally along the input waveguide and also at any point longitudinally along the band-pass output waveguide (i.e., the manifold), provided only that it is positioned at the correct point transversely with respect to each waveguide.
That correct point is anywhere along the respective loci mentioned earlier, where circularly polarized radiation may be (1) tapped off from radiation propagating along the input waveguide, and may be (2) inserted into the output waveguide to reconstruct radiation propagating along the output waveguide. This restriction is very easily met, since it requires only centering a coupling iris at a measured distance from either side of the waveguide.
Thus Nelson's filters perform very well as to the first five considerations outlined in the preceding section. Unfortunately, however, they fail in regard to the sixth.
The Nelson devices are incapable of being tuned to provide elliptic or quasi-elliptic filter functions. Their optimal operation is achieved with tuning to provide a filter function that is known variously as a "Tchebychev," "Tchebyscheff" or "Chebyshef" function--and this function offers less sharp cutoffs than the elliptic or quasi-elliptic functions.
If only the width of the frequency interval of minimum attenuation (maximum transmission) is taken into account, the Tchebychev function provides an adequately narrow passband. The very bottom of the "notch" shape on the attenuation graph is sufficiently narrow, and it is otherwise suitable.
Turning to the shape of the notch at slightly higher attenuation (lower transmission) values, however, the "cutoff characteristic" is found to be unacceptably broad or shallow in profile. With a Tchebychev filter function, excessive power is leaked from each channel into the adjacent frequency regions--introducing either an unacceptably wide guard-band design requirement or excessive crosstalk.
Thus while the short-circuited-manifold technique suffers from inflexible and interdependent positioning requirements, Nelson's configurations suffer from inadequate sharpness of cutoff. It has been well established in the literature that these respective deficiencies are unavoidable intrinsic drawbacks of the operating principles involved in these devices.
The reason, in fact, for inability of the Nelson concept to yield elliptic filtering is closely tied to its very advantages. The input circularly polarized radiation at the entry iris is resolved within the filter cavity into its constituent horizontally and vertically polarized components. In all of Nelson's many designs, the cavity treats these two components identically--and it has appeared that they must be so treated, since they recombine at the exit iris to resynthesize circularly polarized radiation. The resynthesis must be exact to obtain nearly pure circular polarization, and this in turn is required to avoid loss or reflection in the recoupling of circularly polarized radiation out to the output waveguide to reconstruct a wave propagating toward the antenna.
No one has been able to perceive any way of providing bridge couplings for the linearly polarized components within Nelson's unitary cavity, without destroying their characteristic and crucial recombinability. In effect there appears to be a sort of conceptual trap associated with Nelson's appealingly convenient technique of coupling circularly polarized radiation from any point along the source loci: once coupled into the filter, if the circularly polarized radiation is to be resynthesized at an exit iris it is beyond reach, or at least not to be disturbed.
In the literature, however, there appears one other type of directional filter capable of elliptic or quasi-elliptic filter functions. This device is due to Gruner and Williams, who introduced it as "A low-loss multiplexer for satellite earth terminals," Comsat Technical Review 5 157-77 (spring 1975).
Gruner and Williams avoided the seeming trap of the Nelson circular-polarization system, starting instead with a linearly polarized propagating radiation pattern that is frontally collected as it moves through a waveguide. They first direct this wavefront into one port of a device known as a "hybrid" or "quadrature hybrid." This hybrid is used as an input device for the Gruner and Williams filter assembly.
A hybrid is a four-port device which has two key properties. For definiteness of discussion the ports of a hybrid will be identified as ports number one through four. The first essential property of a hybrid is that a wavefront entering at port one is split into two equal wavefronts of different phase, and emitted with a well-defined phase relationship at ports three and four. The device works in reverse as well--that is, two equal wavefronts in correct phase supplied at ports three and four are combined into a single wavefront and emitted at port one.
If wavefronts emitted at ports three and four are reflected, however, by devices placed at these ports, due to the phase reversal in reflection the phase relationship of the two reflected wavefronts is incorrect for return of the power to port one. Rather, and this is the second essential property of a hybrid, the reflected power flows out through the remaining port--port two--of the hybrid.
In the system of Gruner and Williams, the two equal power flows leaving the hybrid separately at ports three and four reach two respective filters, each capable of elliptic or quasi-elliptic function. The broadband power in the stop band is reflected from these filters and leaves the hybrid at port two--where it is absorbed in an attenuator provided for the purpose. The power in the pass band, however, proceeds through the filters. As the filters are identical they preserve the phase relationship between the two wavefronts.
The pass-band output wavefronts from the two filters then enter ports three and four of another hybrid, which for definiteness we will call the "output hybrid." The output hybrid recombines the output wavefronts into a single wavefront having a narrow frequency band, and directs the single wavefront out through port one and into an output waveguide, propagating in a particular direction toward the antenna.
Since the Gruner and Williams system is directional, it has some potential for avoiding the positioning limitations of the short-circuited-manifold technique and therefore is of interest for multiplexer construction. Each channel of such a multiplexer requires an input hybrid and an output hybrid, as well as two complete elliptic-function filter assemblies.
The basic principle of this system is in a very abstract sense analogous to that of Nelson: a propagation direction of a single signal is translated into a phase relationship of two component signals, and the phase relationship is subsequently translated back into a propagation direction for the recombined signal. Between the two translation steps, however, for purposes of bridge-coupling filter procedures there is a crucial difference: the two component signals are inextricably associated with each other and therefore inaccessible in Nelson, but separated and therefore accessible in Gruner and Williams.
In a Gruner and Williams multiplexer the output power from each output hybrid does not proceed directly to the antenna, unless the hybrid under consideration happens to be that one which is geometrically nearest the antenna. The power from any upstream output hybrid is directed instead into port two of a respective adjacent output hybrid. For definiteness this latter will be called the "second hybrid." Since this power is in the stop band of the filters associated with the second hybrid, the power is reflected from the filters and leaves the second hybrid at port one.
As will be recalled, it is port one through which the output power from the filters associated with this second hybrid is emitted. Consequently the power from two channels is combined at port one of the second hybrid. If this power in turn is similarly directed into port two of yet a third output hybrid, adjacent to and further downstream from the second hybrid, the power from three channels will appear at port one of this third hybrid.
Thus there is no combining manifold as such; rather the power flows for the several channels are accumulated by successive passage through the corresponding output hybrids. This system attains two of the principal advantages of directional filters--arbitrary positioning of the hardware for the several channels, and a degree of tuning independence.
There are, however, two serious drawbacks. Although the filter cavities themselves can be made very compact and light by the plural-mode technique mentioned earlier, the hybrids are bulky and heavy. It is for this reason that Gruner and Williams offered their innovation as an "earth terminal." For this reason alone the hybrids would be impractical for satellite applications.
In addition, the hybrids are very costly, and have relatively high dissipation loss--as compared with either the short-circuit technique or the circular-polarization couplings of Nelson. While this loss may be negligible with respect to overall power consumption, it is significant with respect to the spatial distribution of heat dissipation. The cumulative way in which the system collects signals from the several channels by passage through the output hybrids leads to highest power flow in the "downstream" output hybrids. Dissipation is therefore distributed in a very nonuniform fashion, being concentrated in the downstream output hybrids.
Dissipation loss in the output hybrids is also significant with respect to interchannel equalization. The cumulative collection of signals leads to greatest signal loss in the signals from the upstream hybrids. The power level in the signal sources feeding the upstream filters must therefore be adjusted to compensate.
In summary, the Gruner and Williams system satisfies the fifth and sixth considerations mentioned in the preceding section--tuning independence and sharpness of cutoff. In purest theory it also satisfies part of the fourth consideration, weight distribution: the hardware for each channel can be separated by arbitrary distances from the hardware for other channels. This theoretical benefit is not useful, however, since the weight to be distributed is excessive. As to the first three considerations and the other part of the fourth, heat distribution, the Gruner and Williams system is unacceptable for efficient spacecraft design.
No prior system operates satisfatorily with respect to all six considerations outlined above. Weight bulk, and sharpness of cutoff generally have been accorded the highest priority, leading to use of the short-circuited-manifold technique in most modern satellites--despite the associated asymmetry of weight and dissipation, and interdependence of tuning.